The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An engine configured for spark ignition combustion can be adapted to operate in a stratified charge combustion mode under predetermined speed/load operating conditions. It is known that operating lean of stoichiometry using a stratified combustion charge can improve fuel economy but can increase exhaust emissions, including nitrides of oxygen (NOx). It is known to use an ammonia-selective catalytic reduction device to reduce NOx in the presence of a reductant, e.g., urea. It is known that refilling a urea tank can burden an operator.
Known aftertreatment systems for internal combustion engines operating lean of stoichiometry can include a three-way catalytic converter (TWC) followed by other exhaust aftertreatment devices, including a lean-NOx reduction catalyst, also referred to as a lean NOx adsorber (hereafter LNT device) and a selective catalytic reduction (hereafter SCR) device. Known TWCs function to reduce engine-out hydrocarbon (HC), carbon monoxide (CO), and NOx emissions during stoichiometric engine operation and HC and CO emissions during lean operation.
Known SCR devices include catalyst material(s) that promotes the reaction of NOx with a reductant, such as ammonia or urea, to produce nitrogen and water. The reductants may be injected into an exhaust gas feedstream upstream of the SCR device, requiring injection systems, tanks and control schemes. The tanks may require periodic refilling and can freeze in cold climates requiring additional heaters and insulation.
Known catalyst materials used in SCR devices have included vanadium (V) and tungsten (W) on titanium (Ti). Mobile applications include base metals including iron (Fe) or copper (Cu) with a zeolite washcoat as catalyst materials. Material concerns for catalyst materials include temperature operating ranges, thermal durability, and reductant storage efficiency. For mobile applications, SCR devices generally have a preferred operating temperature range of 200° C. to 600° C., and may vary depending on the selected catalyst material(s). The preferred operating temperature range can decrease during or after higher load operations. Temperatures greater than 600° C. may cause reductants to breakthrough and degrade the SCR catalysts, and effectiveness of NOx reduction can decrease at temperatures lower than 200° C.
Known LNT devices adsorb NOx emissions during lean engine operation and operate most effectively within a 250° C. to 450° C. temperature range with effectiveness decreasing above and below that temperature range. The LNT device oxidizes the adsorbed NOx emissions only above a light-off temperature.